Before buying these on eBay why not check out the CGP website?
btw these guides are the best - I don't bother with any others. I used them myself for Maths GCSE and Level 3 and they are really useful for my children. Some bargains on eBay but sometimes it is better to go direct. Watch out for changing syllabus and also don't forget the Levels - there are different books for different levels at KS3 and GCSE.
KS2 pupils should be aiming for a Level 4 but an all rounder with a bit of hard work may be able to achieve a Level 5. Mental maths needs practicing and a bit of searching you should be ble to find the actual transcripts and audio straight off the web. These helped no end with my daughter at KS2.
At KS3 (Year 9) pupils should be working towards a level 6 but higher levels are possible, especially if you use these guides. SATS are now abolished but I shall continue to encourage my childeren to use these guides as the school exams are just as important.